This invention relates to carbon-carbon composite materials.
Reinforced composites are used in a wide variety of applications. The best known composites are made from two-dimensional fabrics and/or fibers dispersed in a resin or plastic matrix. These composites are basically a resin or plastic structure to which reinforcing fabrics or fibers have been added to enhance the physical properties of the structure.
Advances in the field of aerospace technology have created a need for high strength, temperature-resistant materials. For many applications, this need is satisfied by carbon-carbon composite materials.
A wide range of multidirectional reinforced composite structures are now available. The simplest of these structures is obtained by stacking unidirectional fibers or sheets with alternating layers oriented in different directions, or by stacking woven sheets. More complex structures provide three-dimensional reinforcement. The simplest of the three-dimensional structures is the three-directional (3D) structure which generally has reinforcing elements which are mutually orthogonal. The most complex three-dimensional structure is a thirteen-directional (13D) structure. The thirteen directions, with reference to a cube, form three subgroups; the three edges, the four long diagonals, and the six diagonals of the faces.
The reinforced carbon-carbon composite structures are fabricated from graphite or carbon yarn or rods. The term yarn includes continuous filament yarns as well as yarns spun from short fibers, and comprises a plurality of filaments or fibers combined to make up a desired end count. Rods are produced by a pultrusion process whereby unidirectional groups of carbon or graphite yarn are assembled and impregnated with a thermosetting or thermoplastic resin or binder. The impregnated yarn groups are drawn through a die which is warmed to a desired temperature and which has a suitable shape.
The carbon or graphite yarns or rods are assembled into the desired geometric structure. If desired, the yarn may be impregnated with a suitable resin or binder prior to assembly.
The composite is formed either by sintering the reinforcement structure by solidifying the impregnated precursor, thereby avoiding the requirement for other materials, or by the dry or the liquid process, or by a combination of these methods. The dry process consists of depositing pyrolytic carbon inside the structure of the reinforcement by decomposition of a hydrocarbon gas such as methane. In the liquid process, the porous texture of the reinforcement is impregnated with a thermosetting resin or a thermoplastic carbon precursor, such as a phenolic resin, a furanyl resin, petroleum pitch, coal tar pitch, or the like, that is converted to carbon by heat treatment. Following carbonization, the structure is graphitized. The impregnation, carbonization, graphitization cycle is repeated as often as necessary to densify the composite to a desired degree.
The process of densification of the composite generally comprises heat treatment at a temperature in the range of 2500.degree. to 3000.degree. C. and may comprise isostatic pressing at a pressure up to about 15,000 psi in an oxygen-free environment.
Many applications for carbon-carbon composites have been proposed or implemented. The use of such composites for re-entry heat shield applications has been demonstrated. Ehrenreich, U.S. Pat. No. 3,672,936, discloses disk brake pads made of such composites. The use of these materials for turbine disk and blade components, for propulsion system nozzles, thrust chambers, and ramjet combustion liners, and for re-entry vehicle nosetip applications has been investigated.
It is known that the void space of reinforcing structures densified using pitch is incompletely filled. It is believed that volatile components of the pitch form their own escape routes during the carbonization step(s), leaving behind small voids in the densified structure. We have discovered that the size of such voids can be reduced without otherwise altering the bulk density of a densified reinforcing structure.
Accordingly, it is an object of the present invention to provide an improved method for fabricating a densified carbon-carbon reinforcing structure.
Another object of the present invention is to provide an improved pitch for densifying a reinforcing structure.
Other objects and advantages of the invention will be readily apparent to those skilled in the art from a reading of the following detailed description and the appended claims.